Microscopy



April 4, 1939. H. c suoo 2,153,010

MICROSCOPY Y Filed Nov. 25, 19:55 I 10 Sheets-Sheet 1 ATTORNEY.

H. C. SNOOK April 4, 193 9.

MICROSCOPY Filed Nov. 25, 1935 10 Sheets-Sheet 2 INVENTOR.

wigw j ATTORNEY.

April H. c. SNOOK 2,153,010

MICROSCOPY Filed Nov. 25,- 1935 10 Sheets-Sheet s a I I I J llll-illllll1 i 1.4 a 5 I 7 i a: m

I 3: INVENTOR.

v BY

" mamm ATTORNEY.

April 4, 1939. H C SNQ K 2,153,010

MICROSCOPY- Filed Nov. 25, 1935 10 Sheets Sheet 4 [N VEN TOR.

ATTORNEY.

April 4, 1939.

H. c. SNOOK MICROSCOPY Filed NOV. 25, 1935 10 Sheets-Sheet 5 INVENTOR.

ATTORNEY.

H. c; SNOOK MICROSCOPY April; 4, 1939.

Filed Nov. 25, 1955 10 sneets-sh'eet' e JI IJH INVENTOR.

Q .j NV a Maklzii ATTORNEY.

H. C. SNOOK MICROSCOPY April 4, 1939.

Filed Nciv. 25,, 1935 10 Sheets-Shet 7 INVENTOR. M a. JIM- ATTORNEY.

MICROSCOPY Filed Nov. 25, 1935 10 Sheets-Sheet 8 PLATE I v5 INVENTOR. v1M E M T f BY- m w '3 Mia g Q i? ATTORNEY APril 1939- I H. c. SNOOK2,153,010

MICROSCOPY Filed Nov. 25, 1955 10 Sheets-Sheet l0 /Z' 0.5M. M 0.2m.aa/moa/vA [ON/4.

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Patented Apr. 4, 1939 UNITED STATES PATENT OFFICE 12 Claims.

My invention relates to microscopy, and particularly to methods andapparatus for increasing the useful magnification by substantiallyextending the limit of resolution.

In accordance with my invention, for procuring high magnification withmaterially extended limits of resolution, I irradiate the object orspecimen by a selected narrow band of radiation having wavelengthswithin the range from about 2,000 Angstroms to about 200 Angstroms; andfor wavelengths of these orders of magnitudes I utilize animage-producing system comprising, as distinguished from refractors orlenses, refiectors or mirrors having suitably high coefficients ofreflection and suitably low coefficients of absorption.

Further in accordance with my invention, the absorption by ambient airor other gas is reduced or substantially eliminated by maintaining theelements of the optical system in suitably high vacuum, or high vacua,particularly when there are utilize those wavelengths for which theaforesaid absorption is significant; and also for those wavelengths forwhich the ambient air or gas, or for which gas occluded or liberated bythe object or specimen or the vacuum container during microscopythereof, introduces refractive or other undesired effects, such asscintillation; as an alternative, for some types of work, theimage-producing system is maintained in an atmosphere, at any desiredpressure, superor subatmospheric, consisting of a gas or gases, such ashydrogen or helium, having low coefiicients of absorption and refractionat the wavelengths used.

Further in accordance with my invention, one of the mirrors, such as theobjective mirror, comprises annuli whose reflecting surfaces areportions of different ellipsoids having common conjugate foci; moreparticularly, the angular width of each annulus, as viewed from a focus,such as the object-focus, is made sufficiently small materially toreduce aberrations at a conjugate focus, such as the image-focus, tonegligible magnitude.

My invention resides in further aspects of method and apparatushereinafter described.

This application is a continuation-in-part of my copending applicationSerial No. 10,431, filed March 11, 1935.

For an understanding of my invention, and for illustration of apparatusembodying it, reference is to be had to the accompanying drawings, inwhich:

Fig. l diagrammatically illustrates an imageproducing system forproducing photo-micrographic or fiuoro-microscopic images;

Fig. 2 is a side elevation of apparatus having an image-producing systemof the type shown in Fig. l;

Fig. 3 is a plan view of the apparatus of Fig. 2;

Fig. 4 is a detail view, in plan, on enlarged scale and with parts insection, of the primary head of the apparatus shown in Fig. 2-;

Fig. 5 is a view in elevation, taken on line 5-5 of Fig. 4; partly insection and with parts omitted;

Fig. 6 is a side elevation of parts appearing in Fig. 5;

Fig. 7 is a detail view, in elevation and on enlarged scale, of thesecondary head of the apparatus shown in Fig. 2;

Fig. 8 is a plan view of the mirror turret shown in Fig. 7;

Fig. 9 is a detailview taken on line 9--9 of Fig. 10;

Fig. 10 is a sectional view taken on line I0--l0 of Fig. 9;

Fig. 11 diagrammatically illustrates an arrangement for selectivelyusing difierent sources of radiation in the apparatus shown in Figs. 2and 3;

Fig. 12 diagrammatically illustrates a. source of radiation specificallydifferent from that shown in Fig. 1;

Fig. 13 diagrammatically illustrates an arrangement for reducing theoverall length of an apparatus such as shown in Figs. 2 and 3;

Fig. 14 illustrates another modification of the invention;

Fig. 15 is a cross-sectional view of a preferred form of objectivemirror;

Fig. 16 is a graph referred to in explanation of the image-producingsystem;

Fig. 17 illustrates another modification for obtaining wide-angleirradiation of the object.

In the present microscopic practice, the maximum useful magnification isabout 1,000 diameters, and the resolution about 0.11 micron. Althoughhigher magnifications have been employed, the increased magnificationactually gives no greater detail because the resolution is no betterthan aforesaid at 1,000 diameters. At the present time, and with thematerials at present known, greater useful magnification cannot beobtained with the refracting type of microscope. Although it isappreciated the resolution obtainable with a microscope increases as thewavelength of the light or radiation used is decreased, there are knownno suitable refracting materials which are sufficiently transparent toradiation much below 3,000 Angstroms. Radiation of these shortwavelengths can be generated by electronic bombardment of various metalsat various voltages or by condensed sparks and arcs in vacuo, but priorto my invention they have not been used to produce real images in eitherphoto-micrography or fluoromicroscopy.

To obtain substantially enhanced useful magnification and resolution farin excess of those possible with a refracting type of microscope nowemployed, I utilize an image-producing system whose components, asdiagrammatically shown in Fig. l, comprise a suitable source S ofradiations within the range below 2,000 Angstroms, and an arrangementfor selecting a narrow band of wavelengths and impressing it upon theobject or specimen 0. Specifically, the monochromator or apparatus forproviding radiations within a desired narrow band of wave lengths maycomprise, as shown, a source of radiation S, a primary condenser mirrorC, a diffraction grating D and a secondary condensing mirror CI. Theradiation, generated by the source S, which as shown may be a target Tbombarded by electrons emitted by the cathode c, is directed by theprimary condenser C upon the diffraction grating D which is viewed bythe secondary condensing mirror Cl which focuses the narrow band ofradiation selected from the spectrum produced by grating D upon theobject O. The radiation may also be produced by an electric spark or arebetween electrodes E, E of Fig. 12. By suitable selection of thematerial of the target, or of the electrodes, and of the electricvoltage and wave form employed, a band of radiation including thedesired narrow band of substantially monochromatic radiation can beobtained.

The selection of different narrow bands of radiation from the source Sfor irradiation of the object can be effected by varying the angularrelation to each other of any one or more of the elements S, D and C,preferably or most simply by adjustment of D, the diffraction grating.

The object O is at one conjugate focus of the mirror M, whose otherconjugate focus is located in front of the second mirror Ml, preferablyellipsoidal, which views the real image of the object produced by mirrorM and impresses a magnified real image upon the photographic orfluorescent plate P.

It is to be noted that the radiation received by the diffraction gratingD, instead of emanating from a point source or a slit, as is customaryin spectroscopic practice, is collected by the condensing reflector Cthrough the large solid angle or cone of radiation 1'. The irradiationof the object is therefore substantially enhanced. Further, thedifiraction grating D, as below stated, is surfaced with materialselected to have a high coefficient of reflection at the Wavelengthsused. This also affords greater intensity of irradiation of the objectat the wavelengths desired.

The reflecting surfaces of the mirrors M, Ml, C and Cl, and of gratingD, and of any of the other mirrors herein referred to, are of a materialwhich for the band of radiation used has an appreciable coefficient ofreflection and a low absorption coefficient; for example, the surfacesmay be of one or more elementary metals such as aluminum, magnesium,beryllium, cobalt, silicon, sodium, caesium, lithium, potassium, nickelor rhodium, or metal alloys, which may be deposited in any suitablemanner as by sputtering or evaporation. The support, backing or form forthe reflecting surface should be of material having an insubstantialthermal coefficient of expansion, such as fused quartz, invar, Pyrex,etc. The surface of the material of the backing of the mirror is ground,polished and figured after the manner employed in the production ofoptical mirrors. The metallic coating subsequently applied to thesupporting surface may be polished and figured optically, if necessaryor found desirable.

For the range between 2000 Angstroms and 200 Angstroms (one Angstromequals .0001 micron) the effect of air absorption is so great as topreclude transmission of radiation through air at atmospheric pressure.Therefore within this range of wavelengths, or for any wavelengths forwhich occur refractive or other adverse effects of the air or gasbetween elements of the imageproducing system, whether it employsmirrors as in Fig. 1 or is of a refracting type shown in a subsequentmodification, the path of the radiation is in a vacuum preferably highand of the order of Illmillimeters of mercury, or lesser pressures. Asshown in Fig. 1, the entire imageproducing system may be enclosed in ahousing I connected as by the pipe or conduit 2 to a suitable vacuumpump.

As it may be desirable, in some instances, to have a different degree ofvacuum where the radiation is produced, there may be provided an innercasing la which closely fits the path of radiation in the monochromatorto provide a gas path of substantial impedance between the source ofradiation and the major portion of housing I which encloses the mirrorsM, Mi and plate P. Separate pumps may be connected to the outlets 2 andM on opposite sides of the impedance.

The use of mirrors eliminates chromatic aberration, and therefore anynarrow band of a wide range of wavelengths can be used to illuminate theobject without loss of definition. It is usual practice to correct thelens system of a refracting microscope for usually not more than threedifferent wavelengths of visible light, and at all other wavelengthsthere is more or less chromatic aberration. The mirror system,therefore, not only obtains results possible with the known refractivesystems. at the wavelengths for which the refractive elements aresufliciently transparent, but has features of advantage over therefracting type of microscope for still shorter wavelengths. With thelens or refractive system good definition or clarity of the image can beobtained only at the few long wavelengths for which the lens system iscorrected, whereas with a given mirror system good photomicrographs areobtainable at a plurality of related wavelengths most of which are inthe range of wavelengths shorter than visible light. The reflectingmaterials mentioned above are suitable for the range of wavelengthsincluding visible light and extending substantially below 2,000Angstroms. Whereas, with the present types of refracting microscopes themaximum useful magnification is of the order of 1,000 diameters and theresolution obtainable is of the order of 0.11 micron, my method andsystem afford a wide range of useful magnifications, as to a maximum ofupwards of 10,000 diameters and with resolutions better than 0.11micron.

Figs. 2 and 3 illustrate in greater detail the construction of amicroscope system of the type shown in Fig. 1. The tube 3 between thehousing 4 for the source of radiation and the housing 5 for thediffraction grating, the tube 6 between housing and the head H! whichcontains the object, condensing and reflecting mirrors Cl and M, tube 1which connects the primary head HI to secondary head H2 which containsthe second mirror MI, and tube 8 between the secondary head H2 and thehead H3 which encloses a sensitized plate, such as a photographic orfluorescent plate P, are all interconnected to form a single rigid unit,because even slight relative movements of any of the parts of theimageproducing system during exposure would blur the image and therebyprevent realization of the high degree of resolution obtainable with theapparatus. The apparatus as a whole is mounted to be as free from shockas possible as by the damped resilient supports 9 of any suitable type.This is rendered necessary because of the constant seismic disturbancesof the earths crust, as well as other transitory disturbances as causedby railroad trains, automobile trucks, and the like. By making theapparatus as rigid as possible and substantially isolating it from theeffect of seismic and other disturbances, relative motion of the partsand consequent distortion of the image is rendered negligible.

Referring to Figs. 4, 5 and 6 which disclose in detail the internalconstruction of the primary head HI, the radiation from the reflectingmirror Cl is focused upon the object O which is carried by theadjustable support 10, preferably a lazy tongs device mounted within theopening H in the object carrier I2. The object can be moved axially ofthe tube '5 by the lazy tongs [0 which are adjustable externally of theapparatus as by the knob 13, and is adjustable transversely of tube 1 asby the knob l4, and can be moved angularly about the axis of the objectcarrier l2 by adjustment of the knob l5. These adjustments are madewhile the object is being observed through the view telescopes IB andI1, while the object is illuminated either with visible light from themonochromator, as in Fig. 12, or by means of fiuorescense of the objectwhen irradiated by ultra-violet light, wavelengths shorter than 4,000Angstroms, also supplied by the monochromator. When the object isadjusted to proper position with respect to the cross hairs of the viewtelescopes IS and I1, it is then known to be located at the correctconjugate focus of mirror M so that the real image will be located atthe correct corresponding conjugate focus of mirror M properly to beviewed by the second mirror Ml of the head H2.

A plurality of mirrors M may be provided for attainment of differentorders of magnification. The mirrors M are mounted on a common rotatablesupport or turret 18 which is adjustable externally of the apparatus, asby the handle !9, to bring any selected mirror to operative position.The mirrors M are so mounted that each, in turn, when brought into axialalignment with the tube 1 is at approximately the correct distance fromthe object O; i. e. at the position which will produce a real imagealways at the same position within tube 1.

As shown in Figs. 2 and 3, the head H2 is at the other end of tube 1.Its internal construction, as shown in Fig. '7, comprises a turret 20for supporting a plurality of mirrors M! of different focal lengths eachof which is at such distance from the position of the real imageproduced by the mirror M that when brought into axial aligmnent withtube '5 the other conjugate focus of mirror MI is on a photographicplate or fluorescent screen P at the other end of tube 8. The turret 20is adjusted externally of the apparatus as by the handle 2|, thusallowing the operator by various permutations of the mirrors M and MI.to obtain different desired magnifications. For certain lowmagnifications the turret 20 is located to bring the optical fiat 22axially of the tube 5 and the object is adjusted so that the focus ofmirror M conjugate with the object is on the photographic or fluorescentplate located in the head H3, and thereby the final real image is placedon the plate.

The proper position of the real image produced by mirror M for locationin front of any one of the mirrors Ml is ascertained by rotating theturret 20 so that the fluorescent, or focusing, plate 23 is broughtaxially of tube 1. The opening 25 extending from the focusing plate 23through the turret permits the image on the plate to be observed throughthe view telescope 24.

It is to be noted with this construction the positions of the object, ofthe real image and of the photographic or fluorescent plate are fixed.The mirrors in use at a given time have fixed imageproducing positions,and the various magnifications are obtained by selecting the desiredcombinations of mirrors. Each mirror is called upon to perform thesingle task of making a real image for but one set of conjugate foci.This fixation of conjugate foci is of advantage as the mirrors can becorrected for the positions at which each of them is used, avoidingerrors which would occur if different magnifications were sought bychanging the relative distances of the mirrors and object.

To ensure that the distances between the various elements of theimage-producing system and their alignment remain constant, at leastduring the time of use or exposure, the tubes 3, 6, 'l and 8 and theircooperating parts should be of material having a low temperaturecoefiicient of expansion. Alternatively, or in addition, the apparatusmay be enclosed in a housing which is maintained at a constanttemperature during the period of operation.

Since the tubes 3, 6, I and 8 are of fixed length, the maintenance of avacuum, or a desired atmosphere, is facilitated since there is avoidedany need of telescoping or sliding joints to the lengths of these tubes.The selection of the diiferent mirrors at the primary and secondaryheads can be effected without any loss of vacuum or change in the gaspressure or composition in the image-producing system by the features ofconstruction now described.

Referring to Fig. 4, the shaft 21 of the turret l8 and the conicalsealing extension 28 thereof are ground to fit the plate 29 of the headwhich is clamped to the head. housing Eli, the interposed seal or gasketmember 3! preventing leakage at this point. The bearing surfaces arepreferably lubricated with an oil or grease having a very low vaporpressure, such as Apiezon oil or rease. or N-dibutyl phthalate, orbutyl-benzyl phthlate.

The insertion and withdrawal of the object can rier is accomplished withminimum loss of vacuum or gas, because movement of the carrier to theposition affording access to the object moves solid portion of thecarrier to block cornmunica tion between the tube and the outeratmosphere. Specifically, the cylindrical object carrier i2 is ground tofit the tubular casing 32 which is inter posed between the tube '1 andthe primary head. These bearing surfaces are also lubricated with alubricant having a low vapor pressure. To withdraw the object the slideis moved downwardly, as viewed in Fig. 4, or towards the observer, asviewed in Fig. 2, until the upper edge of the aperture H, as viewed inFig. 4, passes beyond the edge 33 of casing 32, thus sealing the end oftube 1. For this position the enclosure now defined by the hole II inthe object carrier and the inside of casing 32 is in communication,through port 34, with suitable means for introducing air into thisspace. The slide then may be fully retracted to bring the object spacewithin the opening ll so that it is external to the apparatus, allowinginsertion or replacement of the object O. The slide may be then returnedto the position bringing the upper edge of the opening ll somewhat belowthe edge 33 of the casing so that the object space is again incommunication with the port 34 allowing the object space either to beevacuated or filled with gas at a desired pressure corresponding tocomposition and pressures within the image-producing system. The slideis then moved to bring the object to proper position axially in tube 1for making of another photomicrograph or visually observable image.

The construction of the secondary head is generally similar to that ofthe primary head. Referring to Fig. '7, the shaft portion 35 for theturret 20 and the conical extension 36 is ground to fit the cooperatingportions of the plate 31 which closes the secondary head. The joint ispreferably lubricated by one of the low pressure lubricants abovementioned. As indicated, the gasket 38 is clamped between plate 3'! andthe cooperating flange of the head housing 39. The Window 49 in plate31, which permits use of the view telescope 24, as above described, isalso suitably sealed in the plate. The several openings 4|, sealed bythe plugs 42, are to permit insertion of suitable tools for adjustingthe several mirrors Ml when initially installed. The difierentcombinations of mirrors in the primary and secondary heads afforddesired difierent degrees of magnification, and the movement of thehandles l9 and 2| to effect a desired combination also brings themirrors to such positions that the real images produced are atsubstantially correct positlons.

The construction for permitting removal and insertion of thephotographic or fluorescent plate P in the head H3 without loss ofvacuum or gas is in general similar to that used in the primary head forallowing removal and insertion of the object 0.

Referring to Figs. 9 and 10 the cylindrical plate carrier 43 is groundto fit the opening 44 extending through the casing 45 which is providedwith an opening 48 in alignment with tube 8. When the plate carrier isin the retracted position shown by full line in Fig. 9, the photographicor fluorescent plate P can be removed from or placed upon the plateholder 41 which is preferably carried by a lazy tongs arrangement 48.The carrier 43 may be moved to the extended position shown by dottedline in Fig. 9, thereby bringing the plate 1? in its holder central withthe axis of the tube B, but with the carrier 43 in a position which isso angularly displaced from the position shown in Fig. 10 that therecess 49 in which the plate holder is disposed is brought intocommunication with the port 50. The valve is then in position to effectconnection to a vacuum pump connected to the pipe 52, or, if desired,the passage 53 may extend to tube 8. After the recess 49 is exhausted orfilled with suitable gas, depending upon the condition under which theimage-producing system is to be used, the carrier 43 is then rotated tobring the plate holder to the position shown in Fig. 10. To remove theplate, the reverse sequence of operations is performed. First, theholder 43 is rotated to bring the space 49 into communication with port50. The valve 5| is then operated to effect communication of this spacewith atmosphere, and then the holder 43 can be withdrawn bringing thesolid portion to the right of the holder, as viewed in Fig. 9, to blockthe end of tube 8. The exposed plate may then be replaced by anunexposed plate. This construction is also useful in oscillographs ofthe type using a photographic plate to record the path of a cathode-raybeam.

With the plate in position to receive the image, it may be uncovered bymoving the shutter actuating knob 54, and is of course recovered beforeremoval of the plate carrier by the same knob. The shutter and itsactuating mechanism are not shown as any of various known arrangementsmay be used. The desired position of the plate axially of tube 8 can beobtained by adjustment of knob 55 which is connected to the lazy tongs,preferably through a suitably calibrated reduction mechanism.

As indicated diagrammatically in Fig. 11, there may be disposed withinthe head 56 of the apparatus shown in Figs. 2 and 3 several sources ofradiation for producing different wavelengths. The handle 51 external tothehead 56 can be adjusted to bring a selected one of the sources infront of the condensing reflector C. One of the sources may be anincandescent lamp, an electric are between metallic electrodes, adisruptive spark discharge between metallic electrodes, as in Fig. 12,or other source of visible light, and the other sources (Fig. 1) may befor producing radiation having a wavelength substantially shorter thanvisible light.

Assuming that a photo-micrograph of a particular object is to be made,the desired source of radiation is brought before the condensing mirror,as above described, and the selected narrow band of wavelengths isbrought to focus at the position of the image by adjustment of themonochromator. The adjustments may be made with the object in positionwhen visible light is employed, or when the object will visiblyfluoresce at the wavelengths used. Adjustments may also be made by useof a calibrated fluorescent screen in the position of the object whichis observed during adjustment of the monochromator to bring the desirednarrow band of wavelengths to the position which the object will occupy.The monochromator may be calibrated, and one or more of the elementsadjustable, the scales cooperating with the adjustable elements beingcalibrated so that when the parts are in predetermined positions thedesired narrow band. of wavelengths will be focused upon the object.

With the source of radiation deenergized the photographic or fluorescentplate can now be inserted and uncovered, as above described. With boththe object and plate in position the source of radiation is thenenergized for a predetermined length of time; the plate is then coveredand removed from the apparatus without loss of vacuum. It is desirablethat a series of photornicrographs be taken with the plates at slightlydifferent positions axially of tube 8 because the focus, particularlyfor radiation of short wavelengths, is sharper than can be determinedvisually. It may also be of advantage to take a series ofphotomicrographs at somewhat different wavelengths to obtain enhancementof contrast between diverse constituents of a heterogenous surface, asin metallography. It is to be understood, of course, that my inventionis not limited to metallography, but comprehends bacteriology,histology, botany, biology, crystallography, and, in general, thedetermination of the fine microscopic structure of all materials.

In many instances it is desirable to hold the temperature of the objectunder observation at a magnitude higher or lower than room temperature.This control can be effected by means included in or associated with theobject carrier. For example, the object carrier may support adjacent theobject a tube provided for circulation of a heating fluid, or arefrigerating fluid such .as liquid air.

The physical dimensions of the apparatus will be determined largely bythe magnifications desired and the construction of the optical system.For example, the apparatus shown in Figs. 2 or 3 may be of the order of20 to 30 feet long. In some cases it may be desirable to reduce theoverall length of the apparatus by folding the optical paths, asindicated in Fig. 13, by use of plane reflecting mirrors between some ofthe optical elements hitherto described. For example, as shown in Fig.13, the plane reflecting mirror Pa may be interposed between the sourceof radiation and the diffraction grating D; the plane reflecting mirrorPi may intervene between the diffraction grating and the secondarycondensing mirror Cl; and the plane reflecting mirrors P2, P3, P4 may bedisposed at other points in the optical path.

The entire system, as in the modification above described, is preferablymaintained in a vacuum or suitable gaseous atmosphere. For microscopy atthe wavelengths below 2,000 Angstroms the surfaces of the plane mirrors,as well as others of the system, should be of material havinginsubstantial absorption coefficient and appreciable coeflicient ofreflection.

In Fig. 14 is diagrammatically shown the optical parts of a microscopeof the re-fracting type modified to obtain some of the advantages of myinvention. Briefly, the radiations from the source S are focused by thecondensing lenses L upon the slit 5'1. The collimator lens Ll collectsthe light from this slit and sends parallel rays to the prism 58. Thelens L2 on the other side of the prism focuses the desired narrow bandof the spectrum produced by the prism upon the slit 59. The lens L3parallelizes the monochromatic light issuing from the slit upon thetransparent plate 60 which reflects light through the objective lensesL4, L5 upon the object O. The light from the object passes through L5,L4, 60, and the last lens L6 of the objective to produce a real image ofthe object in front of the projection eye-pieces consisting of lenses L!which produce a real image on the photographic, fluorescent or visualfocusing plate P. The entire optical system, including the source,monochromator, microscope objectives, projection eye-pieces, and theoptical path through the plate to the photographic plate, is enclosed byhousing I, as in the system of Fig, 1, to permit maintenance of a Vacuumor a suitable gaseous atmosphere, as above described, to avoid suchadverse effects of air, as absorption, scintillation, etc.

A suitable and preferred construction for the objective mirror M of themodifications of Figs. 1 to 13 is shown in cross-section in Fig. 15. Thecurved reflecting surface or vertex mirror V has a circular peripheryand in the particular objective mirror shown extends about 5 on eachside of the axis A. The reflecting surface V, concave toward F, is aportion of an ellipsoid whose first conjugate focus is at F, threeinches from the vertex of mirror M, and whose second conjugate focus isthirty feet from the vertex. The reflecting surfaces e-e9 are annularand are portions of different ellipsoids which have the same commonconjugate foci; F, the object focus, three inches from the vertex ofmirror M and Fl, the image focus, thirty feet from the vertex of mirrorM. For the conjugate foci F and Fl, no spherical aberration is producedby the ellipsoidal reflecting surfaces V, ee9, and under the conditionsof use spherical aberration is negligible in the produced images.

For sharp images free of spherical aberration, the object and imagefields should not be greater than about Since the chord of at a radiusof three inches is 0.0261 inch, whereas the size of the object underinvestigation is usually of the order of 0.001 inch to 0.002 inch, thesize of the object field is well within the limit of tolerance.Furthermore, the image field of mirror M, for an object field of .001inch diameter, has a diameter of 0.120 which, located at Fl, thirty feetfrom the vertex of mirror M, has a diameter which is a small fraction ofthe chord of and, therefore, the size of the image field is also wellwithin the limit of tolerance. Moreover, the image field possessesnegligible spheri cal distortion not only longitudinally at Fl but alsolaterally over the angular extent of the image field.

As a condition precedent for negligible coma, each annular zone of theechelon objective mirror M should produce magnifications from thedifferent extremes of its own surface that differ from each other by anegligible amount. In the mirror specifically shown in Fig. 15, thefirst focal distance for each annulus is 3"- L0.0625 and the maximumdifference in the magnifications is which is negligibly small.

The variation in magnification due to different distances from the axisof various parts of the object is caused by a variation in radial objectdistance from F of i-.002". This variation superimposed upon the focaldistance 3":L0.0625 produces a total maximum variation of L0.0645"giving i2.15% as the total percentage change of the magnifications. Comais, therefore, negligible.

Coma may still further be reduced by making the width of the annularzones of the echelon mirror angularly smaller with respect to F. Eachannular element may be made to correspond to F numbers of any suitablerelative aperture as follows;

F/5= =chord of 1129--angular width of annulus F/10= A =chord of44-angular width of annulus F/15= =chord of 347'angular width of annulusF/20=% =chord of 252'-angular width of annulus In the mirror of Fig. 15the vertex mirror and two of the reflecting annuli are each in angularwidth, corresponding to F/5.'73'7+, and each of the remaining reflectingannuli is 5 in angular width and corresponds to F/11.46 for eachannulus.

As previously stated, the reflecting surfaces of the objective mirrorare, for wavelengths shorter than the wavelengths of visible light, of amaterial, which for the band of radiation used, has an appreciablecoefficient of reflection and low coefficient of absorption. Theparaxial surfaces PA of the echelon mirror may, if desired, be ofnon-reflecting material to avoid diffusion of the radiation from theobject.

The mirror Ml is ellipsoidal with its conjugate foci f:12 inches andfl=30 feet (at the plate P). Because the mirror is ellipsoidal, it isfree from spherical aberration at its foci. The object for mirror MI isthe real image produced by mirror M and which, as above stated, is about0.0120 in diameter. At focus f, the real image subtends an arc of about/2 towards mirror MI, and at fl, the real image produced by mirror Ml onthe plate is about 3.6" in diameter, which subtends an arc of abouttowards mirror Ml. Therefore, the longitudinal and lateral sphericalaberration with respect to mirror Ml is negligible.

To obtain negligible spherical aberration, the relative apertures of themirror elements should not be too great. This condition is satisfied bymirror M which, as above stated, has a relative aperture for threeelements near the axis corresponding to F/5.737+, and for the remainingelements a relative aperture corresponding to F/11.46. The relativeaperture of mirror Ml also satisfies this condition; since f i=12", theeffective aperture is 0.3125" and the total aper- It is appreciated thateven when the angular aperture is very small, the focal surface is,nevertheless, a sphere of radius equal to twice the focal length.However, the second focus of each of mirrors M and MI, at which theproduced images are located, is about thirty feet from the mirror,giving a focal surface with a radius of about sixty feet. Since theproduced image, in the case of mirror M, is only about 0.120" indiameter and, in the case of mirror Ml, is only about 3.6 in diameter,it is, in both instances, sensibly fiat.

Furthermore, since the image and object fields employed extend onlyrelatively small angular distances from the mirror axis in each case,astigmatism, which is produced by sagittal, or nonmeridional rays, isnegligible.

When the echelon objective mirror makes real images from objectsilluminated by white light, destructive interference is produced at thereal image of certain wavelengths, while, with other wavelengths whichform the luminous image, there is cumulative interference. The effect ofthe destructive interference is to decrease the brightness of the image.

If the microscope is to be used with monochromatic radiation, inaddition to the foregoing conditions, the mirror M must be constructedto meet the requirement that the various rays of radiation from F whichare reflected by all the elemental areas of all the annuli, and of thevertex mirror shall arrive at Fl in phase with each other; otherwise,the destructive interference at Fl may destroy the image.

Since the vertex mirror and each of the annuli are parts of ellipsoids,all of the rays reflected by each of the individual elements of any oneellipsoid arrive at Fl in phase with each other, so that the requirementis met, considering any two adjacent annuli, when the rays reflectedfrom the anterior edge of the posterior annulus are in phase with therays reflected from the posterior ed e of the anterior annulus.

This condition may be fulfilled whether or not the radial distance, orthe paraxial distance between the adjacent edges of adjoining annuliequal to a whole or integral number of wave lengths. These radial andparaxial distances may each be some different integral number plus orminus different fractions of a wavelength provided that the focaldistances, F, for the two adjacent edges are in proper relation to thesefractional numbers to ensure that the radiation from the edges are inphase at the posterior edge of the anterior annulus.

This result may be obtained by polishing, or by removal of of materialfrom each annulus (or by addition of material) after its plate has b encemented to the vertex mirror, or to the assembly of previously addedannuli. The result of the polishing, or other work upon the surface ofeach annulus may be observed, the progress of the work controlled, andthe correct condition finally verified by observing the interferencefringes in the monochromatic light from F that may be reflected byrestricted zones at the two adjacent edges of adjoining annuli.

For convenience, visible radiation is used in testing and adjusting theparaxial distances between the adjacent edges of the reflecting annuli;for example, 6003.039 21., one of the spectrum lines of iron, or 6438.47A one of the spectrum lines of cadmium. Assuming, for example, that theadjustment has been made at 6003.039 .3...

it is also correct for wavelengths 3001.519 1., 2001.013 23... 1500.759A, 1200.608 3., 1000.506 A.

857.577 5.. 750.380 5., 667.004 11., etc.; i. e., the wavelengths whoserelation to the chosen wavelength can be expressed by whole numbers. Atany of these wavelengths, the cumulative interference at Fl will thereproduce a real image. Similarly, if the adjustment is made at 6438.4?1.. images are produced at wavelengths of 3219.235 5., 2146.156 A,1609.617 A, 1287.694 5., 1073.078 A, 919.78 A., etc.

The annular reflecting surfaces of the echelon mirror M are employed toincrease the effective numerical aperture of the mirror as an objective.

N. A. (numerical aperture) :71 sin u where n=1 (approx) for a vacuum sinu=sin angular aperture The objective mirror specifically shown in 15 hasan effective numerical aperture of about 0.8; other suitable numericalapertures are Table A Total aperture In general, as appears from Fig.16, the larger the numerical aperture, the greater the resolving powerfor a given wavelength. However, the increase in numerical aperture isattended with increased difficulty in satisfying the other conditionsabove discussed.

The limit of resolution with the best refractor type of microscope atpresent known is shown by the cross K, Fig. 16. At a wavelength of 2750A, the smallest object that can be resolved has a diameter of 0.11micron. Shorter wavelengths cannot be used because of the opacity orabsorption of the quartz-fluorite lens system at the shorterwavelengths. The limit of resolution with another high-grade refractortype microscope is shown by cross Y, Fig. 16. At a wavelength of 3650A., the shortest wavelength used, the smallest object that can beresolved has a diameter of 0.140 micron. Shorter wavelengths cannot beused because of the opacity or absorption of the glass lens system atshorter wavelengths.

With my reflector type of microscope, the limit resolution is greatlyextended, as shown by the curves of Fig. 16, which show the resolutionsobtainable with the mirrors of table A, for a range of wavelengths fromabout 2000 Angstroms to about 200 Angstroms.

As above stated, high magnifying power is useless withoutcorrespondingly high resolving power. With any microscope having anoptical system capable of working at a numerical aperture (N. A.) of0.5, the limit set by diffraction to obtain photographic resolution isthe same as the wavelength; that is, the diameter of the smallest objectresolved equals the wavelength of the radiation used. In the case oflarger numerical apertures, the diameter of the smallest object whichmay be resolved is variously less than the wavelength, as is seen frominspection of Fig. 16. It is, therefore, of interest to ascertainwhether the magnification necessary to make the smallest resolvableobject visible to the human eye is within reason. That it is so, isshown by the following table.

The mirror system which I have specifically described affords amagnification of 3600 diameters which, as appears from the table above,is suitably high to render visible the smallest object that can beresolved at wavelengths approaching 200 Angstroms.

For certain classes of work, for example, in metallurgy, the diametersof the specimen containing the object may be large, for example, of theorder of 0.5 inch. In such case, the optical diameter of the object islimited to about 0.001 or 0.002 by a suitable stop coated with materialwhich is substantially non-reflecting at the wavelength used.Preferably, in such cases, wide angie illumination of the object isused. A suitable arrangement is shown diagrammatically in Fig. 1'7. Theradiation from the monochromator is reflected by the conical mirror Claonto the reflecting surface of the ring mirror Clb which, in

turn, transmits the radiation to the object O. The stop Sh prevents theradiation from impinging upon the remainder of the specimen S. Theobject O is at one conjugate focus of the objective mirror M. Thereflecting surfaces of conical mirror Cla and ring mirror Clb are ofmaterial which, for the wavelengths used, has low absorptive power andsuitably high reflective power.

The remainder of the image-producing system may be the same as abovedescribed.

Certain features of my invention which are herein disclosed but notclaimed, for example the rotatable heads HI, H2, and the construction ofthe objective mirror M (Fig. 15), are claimed in my copendingapplication Serial #229,491, filed September 12, 1938.

What I claim is:

In the art of microscopy, the method of procuring high magnificationwith materially extended limits of resolution which comprisesirradiating the object with a selected narrow band of substantiallymonochromatic radiation within the range of from about 2,000 Angstromsto about 200 Angstroms, and producing solely by reflection a magnifiedreal image of the object.

2. In the art of microscopy, the method of procuring high magnificationwith resolution better than 0.11 micron which comprises irradiating theobject with a selected narrow band of substantially monochromaticradiation, within the range of from about 2,000 Angstroms to about 200Angstroms, and producing a magnified real image of the object solely byreflection from one or more surfaces having suitably high coefficient ofreflection and suitably low coeflicient of absorption at the wavelengthsof said substantially monochromatic radiation.

3. In the art of microscopy, the method of procuring h gh magnificationwith materially extended limits of resolution which comprisesirradiating the object with a selected narrow band of substantiallymonochromatic radiation within the range of from about 2,000 Angstromsto about 200 Angstroms, producing a magnified real image of the objectsolely by multiple reflections from surfaces having suitably highcoeflicients of reflection and suitably low coefficients of absorptionat said wavelengths, and providing for the radiation to said object,from the object to said surfaces, and from said surfaces a path devoidof media having substantial coefficients of absorption and refraction atthe wavelengths of said substantially monochromatic radiation.

4. A microscope for producing a real image of an object comprising meansfor producing radiation within the range of from about 2,000 Angstromsto 200 Angstroms, means for selecting therefrom a narrow band ofwavelengths of substantially monochromatic radiation and directing itupon said object, and means for producing a magnified real image of theobject solely by reflection comprising a reflector having a reflectingsurface of material having a suitably high coefficient of reflection andsuitably low coeflicient of absorption at the wavelengths of saidmonochromatic radiation.

5. A microscope affording magnifications in excess of 1,000 diameterswith resolutions substantially better than 0.11 micron comprising meansfor producing radiation having wavelengths within the range of fromabout 2,000 Angstroms to about 200 Angstroms, means for selecting anarrow band of wavelengths of said radiation for irradiation of theobject, and means for producing a real image of the object solely byreflection comprising a reflector whose reflecting surface is ofmaterial having suitably high coeflicient of reflection and suitably lowcoefficient of absorption at said selected wavelengths.

6. A microscope affording magnifications in excess of 1,000 diameterswith resolutions substantially better than 0.11 micron comprising meansfor producing radiation having wave lengths Within the range of fromabout 2,000 Angstroms to about 200 Angstroms, means for selecting anarrow band of wavelengths of said radiation for irradiation of theobject, means for producing a real image of the object solely byreflection comprising a reflector whose reflecting surface is ofmaterial having suitably high coefficient of reflection and suitably lowcoefiicient of absorption at said selected wavelengths, and means formaintaining the optical path substantially free of gas or gases havingsubstantial refractive or absorptive power at said selected wavelengths.

'7. A microscope affording magnifications in excess of 1,000 diameterswith resolutions substantially better than 0.11 micron comprising meansfor producing radiation having wavelengths within the range of fromabout 2,000 Angstroms to about 200 Angstroms, means for selectingtherefrom a narrow band of wavelengths f substantially monochromaticradiation for irradiation of the object, and means for procuring amagnified image solely by reflection comprising a reflector forproducing a magnified real image of said object, and a second reflectorfor producing a magnified real image of said first real image, saidreflectors having reflecting surfaces of material whose reflectioncoeflicient is suitably high at said selected wavelengths.

8. A microscope affording magnifications in excess of ,000 diameterswith resolutions substantially better than 0.11 micron comprising meansfor producing radiation having wavelengths within the range of fromabout 3,000 Angstroms to about 200 Angstroms, means for selectingtherefrom a narrow band of substantially monochromatic radiation forirradiation of the object, means for producing a real image of theobject solely by reflection comprising a reflector whose reflectingsurface is of material having suitably high coefiicient of reflectionand suitably low coefficient of absorption at the wavelengths of saidmonochromatic radiation, means for maintaining the optical pathsubstantially free of gas or gases having substantial refractive orabsorptive power at the wavelengths of said selected radiation, andmeans permitting insertion and withdrawal of the object into and fromthe optical system without substantial effect upon the gas compositionor gas pressure in said system.

9. The method of obtaining resolution of objects smaller than 0.11micron which comprises illuminating the object by a narrow band ofsubstantially monochromatic radiation within the range of from about2000 Angstroms to 200 Angstroms and producing a magnified real image ofthe object solely by reflection in an optical system including a mirrorhaving a numerical aperture in excess of 0.5.

10. A system comprising a housing, a source of radiation in said housingand for which radiation air has a low transmission power, means forevacuating said housing, a photographic plate for receiving an imageproduced by radiation from said source, and means for introducing saidplate into said housing and for removing it therefrom withoutsubstantial loss of vacuum.

11. In the art of microscopy, the method of procuring magnifications inexcess of 1000 diameters with resolutions substantially better than 0.11micron which comprises, in the absence of visible light, irradiating theobject by radiation of a selected narrow band of wave lengths within therange of from about 2000 Angstroms to about 200 Angstroms, and producingupon a photographic plate solely by reflection a magnified real image ofthe object so irradiated.

12. In the art of microscopy, the method of procuring magnifications inexcess of 1000 diameters with resolutions substantially better than 0.11micron which comprises producing radiation having wave lengths withinthe range of from about 2000 Angstroms to about 200 Angstroms, selectinga narrow band of said radiation, irradiating the object solely by saidselected band, and solely by reflection, producing upon a photographicplate a magnified real image of the object at the wavelengths of saidselected band.

HOMER C. SNOOK.

